For those who may not have clicked on the original video, it was Mickey Mantle "signing in" as a mystery guest on What's My Line.
It always frustrated me when someone would sign as "Mr. X" because their celebrity status was such that you'd know their name but you wouldn't recognize them if you saw them on the street.
One non-sports example:
Brian Epstein, manager of the Beatles. I'd have loved to watch him sign his name.